The mechanisms behind the alteration of chemical carcinogenesis by the dietary lipotropes, choline, methionine, folic acid and vitamin B12 have been studied. The chronic administration of diethylnitrosamine (DEN) to rats led to an increased uptake of labelled precursors into hepatic lecithin, while the uptake of labelled precursors into phosphatidyl ethanolamine and phosphatidyl serine was unaffected. The effects of promoters on phospholipid biosynthesis are being investigated. The elevated methionine requirement of transformed liver epithelia cells appears to be the consequence of more than one biochemical abnormality. The metabolism and carcinogenic activity of ethionine in different species is being compared.